Aims of the construction industry to build passive and zero energy buildings are leading to a situation in which efficient thermal insulation strongly attenuates the signals of mobile phones and other wireless systems, wherein it may be even impossible to use a mobile phone inside a building. There are many reasons for the attenuation, but one reason that has been detected is the use of so called selective glass panes, that is, windows laminated with a conductive coating.
Conventionally, signals of wireless systems can enter buildings through windows, but the conductive coatings may cause an attenuation of even tens of decibels, depending on the frequency range. In addition to the windows, electromagnetic signals have previously been capable of passing through walls of buildings, but aluminium coated thermal insulation boards which are commonly used in walls today, efficiently prevent the propagation of signals into the buildings. Electromagnetic signals may also be attenuated by reinforcements in concrete structures, whereby when passing through such a structure, the signal strength may be reduced too much to be sufficient for the use of e.g. a mobile phone on the other side of the structure.
Attempts have been made to solve this problem by means of, for example, a passive antenna system that comprises two separate antennas and a transfer line connecting these two antennas. Furthermore, such a structure should be provided with a point of transition from each antenna to the transfer line. FIG. 1 shows an example of such a structure. The first antenna 1 can be, for example, a strongly directional Yagi antenna mounted outside a building. This antenna is provided with a transition to a cable 2, for example a solder joint between the central wire of the cable and the radiating element of the antenna. What is essential in this transition point is a change in the electromagnetic field distribution. A TEM waveform passes through the cable, and the currents of the cable are unbalanced. The Yagi current distribution, in turn, is balanced. In many cases, a so-called balun is additionally used in this transition point. A similar transition point is provided at the other end of the cable where the cable is soldered to a second antenna 3 of, for example, patch type, installed inside the building. Such an arrangement has, among other things, the drawback that the outdoor antenna may be difficult to position and may also be visually unaesthetic. These disadvantages are pronounced if several antennas have to be mounted, for example, on the roof of the building to provide a sufficient signal strength. If the apparatus also includes an amplifier 4 and a voltage supply 5 for the amplifier, it is an active system.
Other solutions are known as well, in which at least two separate antennas, transition points between the antenna and a transfer line, and the transfer line connecting the antennas can be identified. For example, international patent application WO 01/45303 A1 presents a module with antennas integrated on opposite faces of the module, and a transfer line placed inside the module, for transmitting electromagnetic signals received by the first antenna to the second antenna. Such a module can be, for example, built in lieu of a conventional brick in a brick wall. A drawback among other things in this solution, too, is that the fitting between the antennas and the transfer line is not lossless. Attenuation of the signal takes place in the transfer line as well.
Document JP 2010159564 presents a vertical ventilation duct for a skyscraper, where a tubular radiator is provided at each floor, intended to act as a transmitter of radio signals between the room and the ventilation duct. Thus, when an antenna, that is, an active element, is placed at one end of the ventilation duct, the signal can be conveyed in the ventilation duct from the antenna to the different floors.
Document JP 2008028549 presents a waveguide arrangement that is slightly similar to that of JP 2010159564, in which e.g. an antenna of a LAN base station can be placed in the waveguide, and wireless terminals can transmit and receive via slots formed in the waveguide. Each embodiment presents a base station antenna placed in a waveguide and thus used as an active element. The waveguide is also provided with one or more tubular waveguides acting as emitters for terminals.